Session variables

PHP
1

Hi. I have many pages whereby I create session variables. The maximum variables set on one page is three. I want to use one php page to handle these variables and send them to my email. Is there anyway I can check which ones are set before sending them to my email? I am trying something like

<?php
session_start();
if(isset($_POST['Submit'])) {

if(isset($_SESSION['sender'])&&!isset($_SESSION['receiver'])&&!isset($_SESSION['message']))
{
	$to = "data@mySite.com";
	$subject = "Card Purchase";
	$sender = $_SESSION['sender'];
	$body = "Senders name " . $sender;
	mail($to, $subject, $body);
}
if(isset($_SESSION['sender'])&&isset($_SESSION['receiver'])&&!isset($_SESSION['message']))
{
	$to = "data@mySite.com";
	$subject = "Card Purchase";
	$sender = $_SESSION['sender'];
	$receiver = $_SESSION['receiver'];
	$body = "Senders name " . $sender . "\
" . "Receivers Name " . $receiver;
	mail($to, $subject, $body);
}
if(isset($_SESSION['sender'])&&isset($_SESSION['receiver'])&&isset($_SESSION['message']))
{
	$to = "data@mySite.com";
	$subject = "Card Purchase";
	$sender = $_SESSION['sender'];
	$receiver = $_SESSION['receiver'];
	$message = $_SESSION['message'];
	$body = "Senders name " . $sender . "\
" . "Receivers Name " . $receiver . "\
" . "Message " . $message;
	mail($to, $subject, $body);
}

} else {

echo "ERROR!";

}
?>

So the first one is
true false false

then true true false

then true true true

2 

foreach ($_SESSION as $key => $value) {
    $body .= " $key $sender\
";
}